This invention relates to construction materials, and more particularly this invention relates to lightweight aggregates and a composition and method for producing the same.
For many years the construction industry has used natural aggregates such as sand, gravel, limestone, and granite in concrete. The industry has found, however, that supplies of these natural aggregates are limited in certain geographic areas and they must be transported into these areas at great cost. Additionally, overall supplies of natural aggregates are rapidly being depleted. Equally important is the fact that with new construction methods and materials, lightweight aggregates have become a necessity. Thus, some lightweight aggregates have been developed to replace natural aggregates in the construction industry. Most of the lightweight aggregates are based on expanded clay and/or shale.
While some prior art expanded clay and/or shale are known and used for construction material lightweight concrete aggregates, their manufacturing constitutes the selection of a particular type and class of shale or clay with inherent properties which exfoliate or expand upon being subjected to high heat in a kiln or sintering operation. These clays will not expand until the mass has reached temperatures above fusion points at which time the material is molten and capable of expansion. Unfortunately, however, all known methods of commercial production subject this material to rolling, tumbling, compacting forces of significant impact, sufficient to neutralize most of the expansion by again compacting the expanded material while it is in the state of fusion, and before it cools.
Additionally, the physical and chemical structure of the raw clay or shale is so consistently variable throughout a given volume that adequate gas generating sources may not be present at all in the chemical structure, therefore, the clay or shale is simply reduced to a compacted ceramic with no value as a lightweight aggregate.
An industry study evidenced that the average cubic yard of clay or shale would provide a finished product of 1.207 cubic yards of loose fill aggregate volume. This amounts to a total gain of 20 percent expansion. Additionally, the fused mass is ejected from the kiln or sintering operation, as large masses of fused (to semi-fused) clinkers which must be impact ground to reduce the large masses to products of small size, which are compatible with cement as a concrete aggregate.
Additional facets of the problem include questionable availability of the basic clays and/or shales of the type which are required, and the proximity of this essential material to the operation. Although some manufacturers appear to have located their operations near adequate reserves, others have learned that their reserves are inadequate and it has become necessary to transport suitable raw clay for great distances in order to support the operation.
Also, adding to the foregoing problems and disadvantages of the prior art, there are considerable variations within the average clay deposit. These variations occur in any deposit from top to bottom, from side to side, and from front to back. The density varies, the chemical composition changes, and as a result the physical characteristics become a constant variable which places a responsibility on the kiln operator that is effectively beyond his control. For example, a variation in the clay chemistry of a nominal 10 percent toward alumina would require the kiln temperatures to be increased by at least 100.degree.F to insure vitrification. Compounding the problem, the kiln operator normally will not be aware of the deviation until the material has already completed the firing cycle. Even assuming that good quality control alerts the kiln operator of an under-fired condition, the kiln response to changes in firing temperature is so slow that a lengthy period of time is required to effect a change in temperature. Thus, the chemical characteristics of the clay could have changed again completely in still another direction before the kiln response factor is complete.
It, therefore, follows that a considerable amount of the commercial product now on the market is actually an averaging of errors and a continuing compromise in quality. A large percentage of the commercial product is not expanded at all and/or is under-fired to the extent that it is relatively soft and very absorptive.
Additionally, kiln operators and supporting labor represent a significant part of the total manufacturing cost of lightweight aggregates. It is, therefore, difficult for the lightweight aggregate to be competitive with natural aggregates which involve an extremely low labor cost. Thus, there is a need for a lightweight aggregate which could be manufactured at a lower unit cost than those presently used.
Comparing the present "bloated clay" lightweight aggregates with the natural, or heavy, aggregates, it is known that the lightweight aggregates will provide a concrete mix of acceptable strength which weigh only 100 to 120 pounds per cubic foot whereas the concrete made with a natural or heavy aggregate weighs 140 to 160 pounds per cubic foot. It has been recognized that this weight differential can be translated into design changes in structures of all types which will yield savings in transporting, handling, placing, and pumping, as well as in the dead weight of the structure which must be supported with steel in many instances.
Additionally, the expanded clay lightweight aggregate has been found to be excellent for road-building as well as for bridge structures. The utility in road-building is enhanced because as the cement wears, exposed aggregate of expanded clay provides excellent traction since it is not self-polishing as is, for example, limestone.
Thus, there is a need for a lightweight aggregate having such physical characteristics as high temperature resistance, high tensile strength, lighter weight with the same strength as present lightweight aggregates, and chemical resistance, as well as being cheaper to manufacture.